In coherent optical communication using homodyne detection, transmission signal frequency at a sender side and a local oscillator frequency at a receiver side have to agree with each other. It is technically difficult to bring light frequencies into complete agreement with each other. In an optical receiver, clock pulses are produced synchronized with data, based upon feedback signals from a digital signal processor, to sample data at the clock frequency. See, for example, Japanese Patent Laid-open Publication No. 2009-60309. However, if a signal is phase-modulated, the phase of the light wave may change depending on the signal component. In this case, a sine wave may not be acquired even if the received signal is mixed with a signal from a local oscillator, and frequency offset between the light source of the sender side and the local oscillator cannot be detected.
Conventional clock extraction circuits are designed under the assumption that a constant envelope phase modulation scheme such as binary phase shift keying (BPSK), differential phase shift keying (DPSK), or quadrature phase shift keying (QPSK) is employed. When using a multi-level phase modulation such as 16 quadrature amplitude modulation (16-QAM) or 64-QAM with multiple levels in the amplitude direction, the multiplication result from a coupler provides multiple values (e.g., four amplitude levels when employing 16-QAM). Accordingly, a clock extraction circuit designed for intensity modulation cannot extract a clock pulse.
Intra-dyne coherent detection tolerates a slight amount of frequency offset between a sender-side light source and a receiver-side local oscillator. See, for example, P. J. Winzer, et al., “56-Gbaud PDM-QPSK: Coherent Detection And 2,500-km Transmission”, ECOC 2009. With intra-dyne coherent detection, symbol rotation (phase rotation) occurs due to a frequency difference between the sender-side light source and the receiver-side local oscillator, as illustrated in FIG. 1A. To compensate for the symbol rotation, a digital signal processor creates an inverse rotation as illustrated in FIG. 1B, and stops the rotation at the symbol positions illustrated in FIG. 1C. If the local oscillator frequency changes during the compensation for the symbol rotation, the frequency difference estimated from the symbol rotation also varies. Consequently, the phase rotation transiently deviates from the frequency offset compensation value estimated at the digital signal processor, and burst error is caused. Besides, when the local oscillator frequency changes, the phase of a data signal also changes and the clock extraction circuit may malfunction.
A technique for broadening a frequency offset compensation range is proposed. See, for example, “Novel Wide-range Frequency Offset Compensator Demonstrated with Real-time Digital coherent Receiver”, H. Nakashima et al., ECOC 2008. This technique is called Pre-decision-based angle differential frequency offset estimator (PADE) algorithm. PADE can broaden the compensation range; however, Q penalty becomes large as the frequency offset increases. It is confirmed that Q-factor penalty occurs even if ideal frequency offset compensation is performed.
It is expected that, in a feature optical network, flexible grid technology with variable frequency-grid intervals or ultimately, gridless technology without frequency grid is widely used. See, for example, “Building a Fully Flexible Optical Layer with Next-Generation ROADMs”, HEAVY READING, Oct. 2011, White Paper. In variable-grid or gridless communications, optical signals cannot be received even if digital signal processing using PADE is employed.
It is desired to provide an optical communication technique that can bring a local oscillator frequency into agreement with or close to a transmitter light source to establish communications and reduce Q-factor penalty due to frequency offset, regardless of a modulation scheme.